Water-based inks for inkjet recording that use pigments as coloring materials are capable of on-demand printing and have many advantages such as good light fastness possessed by the pigments and a reduction in the risk of a fire and mutagenicity that could be caused by organic solvent inks. Therefore, such water-based inks are used not only for plain paper applications but also for wide variety of applications such as coated paper, art paper, plastic films such as vinyl chloride and polyester films, metals, and fabrics. In particular, the recent growth of the print-on-demand market has led to the need for printing on plain paper, coated paper, art paper, plastic films, etc. at a high speed comparable to the speed of planographic printing.
Generally, the drying rate of a water-based ink is lower than that of a solvent ink. In printing on plain paper with a water-based ink for plain paper, the water is absorbed by the paper. However, in printing with the water-based ink for plain paper on a non-water absorptive substrate such as plastic or metal, highly hydrophobic coated or art paper, etc., offset may occur in printed portions during high-speed printing, e.g., during stacking of sheets printed by sheet-fed printing or during winding in roll-to-roll printing. Another problem is that fixability is poor.
In the inkjet recording method, ink droplets are ejected from very fine nozzles of several tens of micrometers to record an image. When an ink with an excessively high drying rate is used, the ink may dry and adhere to the forward ends of the nozzles, and this may adversely affect the ejectability of the ink.
Accordingly, there is a need for water-based inks for inkjet recording excellent in ejectability and having a drying rate adaptive for high-speed printing even when the substrate is non-water absorptive plastic or metal, highly hydrophobic coated or art paper, etc.
One known water-based inkjet ink composition highly suitable for printing on coated paper, art paper, vinyl chloride sheets, etc. and capable of providing high-quality images is a water-based inkjet ink containing a water-soluble organic solvent having a boiling point of from 100° C. to 180° C. inclusive and an organic solvent having a boiling point of from 200° C. to 280° C. inclusive and a surface tension of from 20 mN/m to 30 mN/m inclusive (see, for example, PTL 1).
One known water-based inkjet ink suitable for high-speed printing uses a specific amount of a surfactant in which the average number of moles of ethylene oxide added to acetylene glycol is from 0.5 to 5.0 inclusive and a specific amount of another surfactant in which the average number of moles of ethylene oxide added to acetylene glycol is from 8.0 to 35.0 inclusive at a specific ratio (see, for example, PTL 2).
In PTL 1, drying properties are evaluated as follows. Printing is performed on a printer using different numbers of printing passes, and whether or not mottling occurs at the minimum number of passes is checked. However, even with an ink having high resistance to mottling (a trouble that occurs when the ink does not adhere to a solid printed portion (a portion fully covered with the ink) uniformly and forms a nonuniform printed surface), the above offset problem that occurs during high-speed printing has not yet been solved in all cases. In PTL 2, offset in printed portions is not studied. Moreover, in the above literature, ejection failure caused by drying of the ink is not studied.